1. Field of the Invention
The present invention relates to techniques for predicting rotor winding failure, and predicting stator winding failure; and particularly to techniques which utilize current zero crossing times of stator currents to predict stator and rotor winding failure.
2. Description of the Art
One type of commonly designed induction motor is a three-phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC-voltage source by a separate supply line, the source generating an alternating current therein.
As the stator currents alternate, a wave of magnetic field flux, directed radially toward the rotor, rotates around the axis of the motor. The relative motion between the stator flux wave and the rotor windings induces an alternating voltage and current in each of the rotor windings. The alternating rotor currents, in turn, produce a magnetic rotor flux, directed radially toward the stator. Because of the interaction between the flux fields, the rotor experiences a force tending to rotate the rotor as the stator currents alternate.
While modern power stations supply well-balanced three-phase voltages having identical periods and amplitudes and having phases which differ by exactly 120 degrees, it is not uncommon for the voltages at the point of utilization in AC motors to be unbalanced. Unbalanced stator terminal voltages, and the resulting unbalanced stator currents, are generally recognized as undesirable for a plurality of reasons.
Asymmetrical stator and rotor windings are two sources of unbalanced stator winding voltages and currents. While great effort is taken to manufacture symmetrical stator and rotor windings, over the course of time, windings can become asymmetrical as their properties change though heavy industrial 10 , use. Asymmetrical stator windings produce unbalanced currents through the three stator windings.
Even where stator windings are symmetrical, if the rotor windings become asymmetrical, the rotor flux will be unbalanced. As known in the art, rotor flux induces back EMFs in the stator windings. If the rotor flux is unbalanced, the back EMFs are also unbalanced and influence the stator currents. Thus, asymmetrical rotor windings can act as a second source of stator current unbalance.
Importantly, even a small supply voltage unbalance can result in a relatively large current unbalance in the stator windings of a three-phase motor. This phenomenon can be understood by representing an unbalanced voltage as a combination of a normal three-phase positive sequence voltage component, plus a small negative sequence voltage, which on its own would drive the motor in the reverse direction.
As well known in the art, the stator windings present only a very small impedance to the negative sequence voltage component. As current equals voltage divided by impedance, dividing the negative sequence voltage by a small impedance produces a large negative current component. In some cases, where there is a 3% voltage unbalance, as a result of the negative current component the stator current unbalance might be as high as 18%-24%.
Unbalanced currents deliver uneven power to the rotor and thus produce undesirable torque pulsations and motor vibrations. The vibrational forces caused by the vibrating motor accelerate deterioration of the mechanical components of the motor including the rotor and stator windings. If the motor is operating at or near its fully-rated load, the rotor and the stator windings carrying increased current heat up unnecessarily. While extreme overheating may trip an overload relay to switch off the motor and protect it from burning out, lesser degrees of unbalance usually go unchecked as the heat generated is insufficient to trip the overload relay.
Excess heat causes motor insulation to age at an accelerated rate and causes accelerated deterioration and evaporation of the bearing and other lubricants both of which shorten the useful life of a motor. In addition, the excess motor heat is lost energy which means the motor is running inefficiently.
Thus, predicting stator winding and rotor winding faults is important in order to prolong the useful life of a motor, and to prevent unexpected motor failure which may interrupt important processes. However, predicting winding failure before catastrophic failure occurs is complicated by the fact that winding deterioration is a slow, gradual process up to a point, after which there may be a sharp increase in asymmetry and thus a sharp increase in the potential for motor damage. If winding failure can be detected at an early stage, preventative maintenance can be scheduled, and causes of failure investigated, at convenient times, without disrupting operations.
Because the effects of stator and rotor failure are not easily observed, in many instances, failure cannot be detected until after costly motor damage has resulted. The motor control industry has not come up with a simple way to discover the initial signs of eventual winding failure. Because most industrial size stators and rotors have complex winding patterns, it is not practical to predict motor failure from a visual inspection of the windings. Furthermore, even where a motor employs relatively simple winding configurations, it would be extremely burdensome for a user to dismantle a motor on a regular basis in order to inspect the windings for damage.
One way to predict stator and rotor winding failure might be to use sensors which could track stator current unbalance to discover the initial signs of failure. However, an elementary measurement of the magnitude of current unbalance is not helpful as there are many sources of unbalance in addition to winding asymmetry (i.e. non-uniform distribution of electrical load, unmatched transmission impedance between the source and the motor . . . ). The source to which unbalance is attributable cannot be deciphered simply by tracking the extent of unbalance.
Therefore, it would be helpful to have a method and/or an apparatus for use with a motor controller which could predict stator winding and rotor winding failure at an early stage of deterioration using the computational abilities of the motor controller.